For many years, optical modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. A continuous wave (CW) optical signal is launched into the waveguide, and an electrical data signal input is applied as an input to the metal contact regions. The applied electrical signal modifies the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Various other conventional electro-optic devices are similarly of a material (such as III-V compounds) that are not directly compatible with a silicon platform.
A significant advance has been made in the ability to provide optical modulation in a silicon-based platform, as disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005, assigned to the assignee of this application and incorporated herein by reference. FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in the Montgomery et al. patent. In this case, silicon-based optical modulator 1 comprises a doped silicon layer 2 (typically, polysilicon) disposed in an overlapped arrangement with an oppositely-doped portion of a sub-micron thick silicon surface layer 3 (often referred to in the art as an SOI layer). SOI layer 3 is shown as the surface layer of a conventional silicon-on-insulator (SOI) structure 4, which further includes a silicon substrate 5 and a buried oxide layer 6. Importantly, a relatively thin dielectric layer 7 (such as, for example, silicon dioxide, silicon nitride, potassium oxide, bismuth oxide, hafnium oxide, or other high-dielectric-constant electrical insulating material) is disposed along the overlapped region between SOI layer 3 and doped polysilicon layer 2. The overlapped area defined by polysilicon layer 2, dielectric 7 and SOI layer 3 defines the “active region” of optical modulator 1. In one embodiment, polysilicon layer 2 may be p-doped and SOI layer 3 may be n-doped; the complementary doping arrangement (i.e., n-doped polysilicon layer 2 and p-doped SOI layer 3) may also be utilized.
FIG. 2 is an enlarged view of the active region of modulator 1, illustrating the optical intensity associated with a signal propagating through the structure (in a direction perpendicular to the paper) and also illustrating the width W of the overlap between polysilicon layer 2 and SOI layer 3. In operation, free carriers will accumulate and deplete on either side of dielectric 7 as a function of the voltages applied to SOI layer 3 and doped polysilicon layer 2. The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide defined by the active region. In the diagram of FIG. 2, the optical signal will propagate along the y-axis, in the direction perpendicular to the paper.
While considered a significant advance in the state of the art over lithium niobate modulators, silicon-based optical modulators in general and the exemplary configuration of FIG. 1 in particular are known to suffer from chirp as a result of the inherent phase response and optical loss differences between the two arms of the modulator. Chirp is a time-varying optical phase that can be detrimental to the transmission behavior of an optical signal as it propagates through dispersive fiber. The chirp behavior of optical modulators is often characterized using an “alpha parameter” that is defined as the amount of phase modulation normalized to the amount of intensity modulation produced by the modulator. The alpha (α) parameter may be defined as follows:
  α  =      2    ⁢                            ⅆ          ϕ                          ⅆ          t                                      1          P                ⁢                              ⅆ                          P              ′                                            ⅆ            t                              and may exhibit a value that is zero, positive or negative. In some applications, it is desirable to have a small amount of negative chirp (i.e., a small negative alpha parameter) to extend the transmission distance of a signal along a dispersive medium, such as an optical fiber, before dispersion limits the range.
Conventional silicon-based optical modulators are known to exhibit non-zero chirp (even when configured in a symmetric drive arrangement) as a result of the nonlinear phase versus “applied voltage” response of their structure. Increasing either the modulation speed or the distance traveled by the modulated optical signal have been found to only exacerbate the chirp problem, since the dispersion characteristics of the transmission fiber will have an even greater impact.
FIG. 3 is a plot of phase modulation of a silicon-based optical modulator as a function of applied voltage for the prior art device of FIG. 1. In a cross-coupled MZI arrangement utilizing this particular prior art device, the drive voltage is defined to swing between the values of −1.3 V and +1.3V (these values associated with the particulars of the electronics used as the driver circuit for the modulator), crossing from the depletion region of the modulating device into the accumulation region of the device. As shown, the change in phase modulation over the −/+1.3V range (Δ phase mod) is relatively small, due primarily to the relatively weak response in the depletion region of the device. This minimal phase change in the depletion region results in limiting the modulation efficiency of the prior art structure of FIG. 1. It is also to be understood that the voltage swing −/+1.3V is exemplary only and associated with certain design parameters; various other voltage ranges may also be employed in similar prior art devices.
Thus, a need remains in the art for a silicon-based optical modulator with improved modulation efficiency, extinction ratio and control of its chirp parameter.